1. Field of Invention
The present invention relates to a method of fabricating a recess channel transistor. More particularly, the present invention relates to a method to form a thicker oxide layer on the sidewalls of a trench.
2. Description of Related Art
With increased integrated circuit development, sub-micron generation is developed and the device sizes are becoming smaller. However, reduction of the device size generates short channel effects resulting from smaller channel lengths.
Thus, a recess channel array transistor (RCAT) was proposed to overcome this problem. In comparison with traditional metal oxide semiconductor transistors, the recess channel transistor can reduce short channel effects. Switch speed of the transistor is related to thickness of a gate oxide in the recess channel transistor and thereby the thickness of the gate oxide on the trench is an important design. Generally speaking, the transistor speed increases when the gate oxide formed on the sidewalls, which overlap a source/drain, are thicker and the gate oxide formed on the bottom layer is thinner.
Traditional recess channel array transistor fabrication method initially requires a trench to be formed in a substrate. Thermal oxidation caused by a rapid thermal process forms a gate oxide on the sidewalls of the trench. However, the difficulty of the traditional method is that a thicker gate oxide on the sidewalls of the trench cannot be obtained because the thickness of the gate oxide in the traditional method is uniform. Therefore, the switch speed would be decreased, and problems for gate induced drain leakage (GIDL) and parasitic capacitance would be produced. To form a thicker gate oxide on the sidewalls of the trench and a thinner gate oxide on the bottom of the trench, several thermal oxidation processes need to be performed and thus the manufacturing process becomes complicated and inconvenient.
Thus, there is a need to provide a method to fabricate a recess channel transistor to solve these problems.